Suffocation
A creature can hold its breath for a number of rounds equal to 5 + Constitution modifier, or double this duration if they take a Deep Breath immediately before beginning to hold their breath.
Each round a creature takes reduce their remaining air by 1 round or 2 rounds if they are attacked or take strenuous actions that round. They also lose 1 round worth of air each time they are critically hit or critically fail a resistance roll.
If a creature speaks, uses a breath weapon, or otherwise uses their mouth to expel anything or use verbal components, they lose all remaining air.
When a creature runs out of air, they start suffocating taking 10d10 nonlethal stamina damage every round until they're able to breathe again. They are also Off-guard 1, Slowed 1, and Staggered 1 while suffocating.